The variable pulley transmission, or continuously variable transmission (CVT) as it is commonly called, has been under development for over two decades, but its use as a power transmission is mainly for automotive applications. Its control is rather complicated and is very sensitive to operating conditions. For example, in a control arrangement for a variable pulley transmission disclosed in U.S. Pat. No. 4,458,318, a variable line pressure is controlled and applied to the sheave of the secondary or driven pulley of the transmission, and also serves as the hydraulic power supply to a ratio control valve in a speed control loop of the transmission.
Due to the variable supply pressure in the continuously variable transmission of U.S. Pat. No. 4,458,318, both the pressure control valve (a pressure relief type valve) and the ratio control valve gains change with the supply pressure, and the control is therefore very sensitive to system operating conditions. There is a need for an improved continuously variable transmission with a control arrangement and method for reduction of belt slippage in a continuously variable transmission which overcome these drawbacks and disadvantages of this known continuously variable transmission. More particularly, there is a need for an improved continuously variable transmission with a control arrangement and method which are simpler and more robust than those disclosed in U.S. Pat. No. 4,458,318.
An object of the invention disclosed in the co-pending, commonly assigned U.S. patent application Ser. No. 09/222,958, for CONTINUOUSLY VARIABLE TRANSMISSION WITH CONTROL ARRANGEMENT AND METHOD FOR REDUCING TRANSMISSION BELT SLIPPAGE filed concurrently with this application and hereby incorporated by reference, is to provide a continuously variable transmission with a control arrangement and method which meet this need and which are useful as a constant speed drive for an aircraft electrical power generator wherein belt slippage can be minimized.
In the application of a continuously variable transmission (CVT) as a constant speed drive for electrical power generation, if a large load is suddenly applied, the belt may start to slip. Once the belt starts to slip, due to a change of coefficient of friction between the belt and pulley, the belt will keep slipping and the normal speed control becomes ineffective. A lasting slipping will also cause damage of the belt and pulley. The present invention relates to an improved CVT and method for adjusting the CVT ratio when belt slipping occurs to match the calculated belt speeds at the primary and secondary sheaves so that the normal speed control can be recovered. The invention can be used in combination with the invention of the aforementioned application Ser. No. 09/222,958, or independently.
The continuously variable transmission for transferring drive from an engine to a device to be driven according to the present invention comprises a primary pulley for receiving drive from an engine, a belt, a secondary pulley which is coupled over the belt to the primary pulley for transferring drive to a device to be driven. The primary and secondary pulleys each have an axially movable sheave and a hydraulically operated actuator therefor to effect ratio change of the transmission and to maintain belt tension. A hydraulic pressure control loop controls the hydraulic pressure applied to the actuator of the secondary pulley as a function of the sensed load of the device to be driven on the transmission and the pitch radius of the secondary pulley. An output speed control loop controls the output speed of the transmission driving the device to be driven during normal operation of the transmission. A belt speed matching loop overrides the output speed control loop for adjusting the transmission ratio when a condition of belt slipping occurs to recover from belt slipping.
According to the disclosed embodiment, the belt speed matching loop includes means for detecting a condition of belt slipping in the transmission, and means for adjusting the transmission ratio to recover from the detected belt slipping. The means for detecting a condition of belt slipping estimates the belt speeds at the primary and secondary sheaves and compares the two estimated belt speeds. A condition of belt slipping is indicated when a difference between the two estimated belt speeds exceeds a first predetermined threshold value. The means for detecting measures the primary sheave linear displacement, engine speed and the speed of the device to be driven, and calculates the primary pulley pitch radius and the secondary pulley pitch radius for estimating the belt speeds at the two sheaves in the manner disclosed in more detail hereinafter. More particularly, the means for detecting calculates the estimated belt speed at the primary sheave as a function of the calculated primary pulley pitch radius, the measured engine speed, and an engine side gear ratio, and the estimated belt speed at the secondary sheave is calculated as a function of the calculated secondary pulley pitch radius, the measured speed of the device to be driven, and a device-to-be-driven side gear ratio. The means for adjusting the transmission ratio to recover from belt slipping reduces the difference between the two estimated belt speeds to less than a second predetermined threshold after which normal operation of the output speed of the transmission is resumed by the output speed control loop.
The present invention further includes a method of recovering from belt slippage after the belt starts slipping in a continuously variable transmission for transferring drive from an engine to a device to be driven under varying load conditions. According to the invention, the method comprises detecting a condition of belt slipping in the transmission and adjusting the transmission ratio to recover from belt slipping. In a disclosed form of the invention, the device being driven is an electric generator which is driven at a constant speed by the engine and transmission.
These and other objects, features and advantages of the present invention will become more apparent from the following detailed description of the invention taken with the accompanying drawings depicting a preferred embodiment in accordance with the invention.